Preheat the oven to 400 degrees F (200 degrees C).
Place the potatoes in a single layer in a ceramic casserole dish. Melt the butter in a skillet over medium heat. Add garlic, and cook until golden. Stir in the rosemary, and cook just until fragrant. Pour over the potatoes in the dish. Season with sea salt and pepper.
Roast uncovered for about 30 minutes in the preheated oven, until the potatoes are fork tender. Remove from the oven, and toss with the spinach leaves. Return to the oven for 1 to 2 minutes, until the spinach has wilted.